Scaling UV shells when stitching
You can do it using Live Unwrap and pinning vertices of the islands.

First select one UV island and hit V button to pin all the vertices, then select and pin one vertex in the center of the second island.



Then remove UV seam in the place where the islands should connect.

Change the mode to Live Unwrap.



And grab the center pinned vertex of the island and move it until you have a desired result.